This invention relates to improved signal generators to be incorporated in electromagnetic flowmeters.
An electromagnetic flowmeter is constituted by a signal generator and a converter. The generator includes a flow tube having an insulated lining through which the fluid to be metered flows, the fluid intercepting a magnetic field established in the flow tube by an excitation coil to induce an electromotive force signal in a pair of electrodes mounted on the tube.
The electromotive force signal thereby produced is in accordance with the magnetic field intensity and the flow rate of the fluid, this signal being converted by a converter coupled to the signal generator into a current that lies within a predetermined range to provide a flow rate output.
The excitation coil of the signal generator is excited by a current derived from an external power supply that is included in the converter or at a remote point. Since a deviation in this excitation current causes a measurement error, this current is usually set in the converter or at the remote point.
However, the diameter of the flow tube of the signal generator in a flowmeter depends on the flow rate to be measured, different diameters being used for various applications. The set value of the excitation current varies accordingly, so that it is general practice in the prior art to install the signal generator at the measurement site and then set the excitation current. It is therefore necessary to re-set the excitation current each time a given signal generator is replaced with a new generator.
That is, in the prior art there is a lack of replaceability with respect to the relationship between the signal generator and the converter. This gives rise to the disadvantage that even if the converter remains unchanged, it is necessary to set the excitation current in the field each time a new signal generator is combined with the converter. Should the field setting of the excitation current be incorrect, this results in a measurement error.